Display screen or portion thereof with a transitional graphical user interface



FIG. 1 is a front view of the first image of a display screen or portion thereof with a transitional graphical user interface showing the new design.

FIG. 2 is a second image in a sequence thereof; and,

FIG. 3 is a third image in a sequence thereof.

Within the drawings, the appearance of the transitional image sequentially transitions between the embodiments shown in FIG. 1 through FIG. 3. No ornamental aspects are associated with the process or period in which one image transitions to another image. The outermost broken lines illustrate the display screen or portion thereof and form no part of the claimed design. The remaining broken lines illustrate portions of the graphical user interface and form no part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with a transitional graphical user interface showing the new design, as shown and described. 